It is known in the ICD art to provide a "tiered" therapy with regard to the termination of sensed tachyarrhythmias. The term "tiered" therapy has been used typically to describe the different tachyarrhythmia rate zones, such as low rate ventricular tachycardia (VT Low), high rate tachycardia (VT High), and ventricular fibrillation (VF). The term "tiered therapy" also has been used to describe the increasing degree of aggressiveness within each rate zone. For example, within each zone the physician may program the number of stimulation pulses, the interval between stimulation pulses, the energy level of the stimulation, and the number of attempts, etc. Thus, based upon the detected rate of the arrhythmia, the ICD will continue to increase the level of aggressiveness until such arrhythmia is terminated.
For example, upon sensing a low rate ventricular tachycardia, an ICD may attempt to terminate such tachycardia by first applying a prescribed type of antitachycardia ("antitach") pacing. Such antitach pacing typically includes burst pacing, ramp pacing, and/or scanning pacing, as is known in the art. See, e.g., U.S. Pat. Nos. 4,427,011 and 4,541,430 (burst pacing); 4,398,536 (ramp pacing); and 5,103,822 (scanning pacing); which patents are incorporated herein by reference. If the antitach pacing is unsuccessful, the ICD may be programmed to apply a low energy, cardioversion energy shock in an attempt to cardiovert the heart. If the low energy cardioversion shock is unsuccessful, the ICD may apply a higher energy shock in an attempt to cardiovert the heart. If the tachycardia accelerates to ventricular fibrillation, the system will then apply a high energy defibrillation shock with starting values typically on the order of 10 joules and increasing up to 40 joules.
It should be noted that the basic difference between cardioversion and defibrillation is the type of arrhythmia being detected. That is, the term "cardiovert" or "cardioversion" refers to the application of energy shock treatment to a heart in response to a sensed ventricular tachycardia in an attempt to terminate the tachycardia. The term "defibrillate" or "defibrillation," on the other hand, refers to the application of energy shock treatment to a heart in response to a sensed ventricular fibrillation in an attempt to terminate the fibrillation. Cardioversion is often thought of as being "low energy" since the heart has been known to successfully convert the arrhythmia with electrical shocks in the range of 0.1 to 10 joules. However, it is still classified as cardioversion if the arrhythmia is a ventricular tachycardia even if the energy required to convert the arrhythmia goes up to the maximum value (e.g., 40 joules). Defibrillation, on the other hand, is often thought of as being "high energy" since the heart has been successfully defibrillated with energy shocks in the range of 10 to 40 joules. In contrast, the pacing pulses that are applied to a heart during normal or antitach pacing are typically of much lower energy (e.g., between 50 and 200 micro joules). The principal difference between the types of therapy provided by an ICD supporting tiered therapy is that of arrhythmia detection and the programmed level of aggressiveness with the therapy typically starting by applying the lowest energy stimulation for that detected arrhythmia and working its way up to high energy shock therapy, as required.
The energy shock, whether cardioversion or defibrillation, is delivered to the heart via what is commonly referred to as "defib leads," "defibrillation lead or electrode," or "patch electrodes." However, it is recognized that the electrode system could also be epicardial electrodes (attached to the external surface of the heart) or endocardial (attached to the internal surface of the heart) or any combination of patch, epicardial or endocardial. Such electrodes are well known in the art. See, for example, U.S. Pat. No. 4,662,377 (Heilman et al.), issued May 5, 1987, entitled "Cardioverting Method and Apparatus Utilizing Catheters and Patch Electrodes"; U.S. Pat. No. 4,481,953 (Gold et al.), issued Nov. 13, 1984, entitled "Endocardial Lead Having Helically Wound Ribbon Electrode"; and U.S. Pat. No. 4,010,758 (Rockland et al.), issued Mar. 8, 1977, entitled "Bipolar Body Tissue Electrode," which patents are hereby incorporated herein by reference. Hereinafter, the electrodes (whether patch, epicardial, or endocardial, etc.) will be referred to as simply "shocking electrodes."
In order to apply an electrical pulse to the heart (whether of low, moderate or high energy), it is first necessary to charge one or more output capacitors of the ICD device with an electrical charge of the desired energy. Typically, one set of low voltage output capacitors (which may be a single capacitor) is charged to provide the requisite energy for normal or antitach pacing. The normal pacing lead(s) is then A.C. coupled to such set of low voltage capacitors through an appropriate output switch. Another set of capacitors, which is generally a set of high voltage capacitors, is charged to provide the requisite energy for cardioversion or defibrillation. Appropriate shocking electrodes are then coupled to such set of high voltage capacitors through an appropriate high voltage output switch. When an electrical stimulation pulse is to be applied to the heart, the appropriate output switch is closed to connect the low or high voltage output capacitor(s) to the cardiac tissue through either the pacing or shocking electrodes, thereby effectively "dumping" the charge stored in the low or high voltage output capacitor(s) across the cardiac tissue.
All tiered therapy ICD's known to applicants begin to charge their output capacitor(s) following verification of the failure of the previous less aggressive therapy. For example, as soon as the ICD sensing and logic circuits determine that the first tier of therapy (e.g., antitach therapy) has not successfully terminated the tachyarrhythmia, the high voltage output capacitors are charged to the appropriate energy for a second tier of therapy (e.g., cardioversion in the case of ventricular tachycardia; or defibrillation in the case of ventricular fibrillation). As soon as the ICD sensing and logic circuits determine that the second tier of therapy has not successfully terminated the tachyarrhythmia, the high voltage output capacitors are charged to a higher energy level. Unfortunately, it may take 3-4 seconds to charge the high voltage output capacitors to a moderate energy level to (1 to 10 joules), and 7-15 seconds to charge such capacitors to a high energy level (11 to 40). Disadvantageously, these charging times represent a significant period of time, or "time-to-therapy," during which the tachyarrhythmia continues without the benefit of having the ICD apply any therapy. Furthermore, as the duration of the tachyarrhythmia lengthens, it becomes increasingly more difficult to terminate the arrhythmia with an electrical stimulus. Thus, what is needed is an ICD device wherein the time-to-therapy is reduced, thereby reducing the exposure of the patient to the tachyarrhythmia, and thereby increasing the probability of success of the cardioversion-defibrillation attempt.